Hi Vito:
> Could either of you clarify why a double insulated SELV circuit can not be
> grounded? Is it a 60950 based or based on some other performance(?) based
> requirements?
This requires a bit of history. I don't have all of
the history, but I can fill in some of the blanks.
IEC 536 (1976) is the base document for equipment
classes, i.e., I, II, and III. Class III required
the introduction of SELV.
IEC 536 was prepared by the same folks who wrote the
IEC 364 series of standards on building installations.
For these folks, SELV was presumed not to be connected
to ground. (If they had been in the USA, these folks
would have considered the doorbell circuit as SELV.)
The standard did not explicity prohibit SELV from
being grounded. The committee was quite shocked to
learn that equipment manufacturers were using the
concept for low-voltage secondary circuits and were
grounding one pole of the SELV. They did have a
technical explanation of why grounding SELV was not
acceptable (which I have forgotten and, at the time,
I considered faulty).
In revising IEC 536, the committee then considered the
issue of grounding SELV. They decided to retain SELV
as ungrounded, and introduced a new concept, PELV
(protected extra-low voltage) which was protected by
virture of being grounded. They also introduced FELV
(functionally-grounded extra-low voltage).
The IEC 950 folks ignored IEC 536 -- sort of. They
did not adopt PELV, but retained SELV. Instead, they
did provide for PELV by specifying "Method 3" under
SELV circuit requirements. (At one time, several
European countries did not accept Method 3.) The IEC
950 definition of SELV includes a note: "This
definition of SELV CIRCUIT differs from the term SELV
as used in IEC 364."
A SELV circuit can be grounded. As a result of
connecting the SELV to ground, the name of the circuit
may change to PELV or FELV, depending on the end-
product standard. If the end-product standard is
IEC 60960, then the circuit name remains SELV.
Best regards,
Rich
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